The preterminal breaths during asphyxiation, termed "asphyxial gasps" or "secondary gasping," are believed to be an indication of impending death when observed in the fetus in utero. In postnatal life, asphyxial gasping can be the mechanism of spontaneous recovery ("autoresuscitation") from prolonged apneic spells. The proposed research is designed to investigate the neural regulation of asphyxial gasping in the newborn and fetal rabbit. The first phase of the work will be a detailed description of the gasping pattern of respiration in terms of frequency, force and inspiratory and expiratory durations. Gasping produced by acute airway occlusion will be compared to gasping observed during circulatory failure and gasping as it occurs during hypercapneic hypoxia produced by rebreathing. A description of gross phrenic nerve activity and the activity of single phrenic fibers is planned. This will be related to the diaphragmatic EMG and occluded airway pressure during gasping. In the second phase of the work, the influences on the gasping pattern of peripheral chemoreceptors, vagal nerves, laryngeal and cutaneous neural afferent activity will be investigated. Additionally, experiments combining brain stem transection with asphyxiation will examine the contributions of supramedullary inputs to the gasping pattern. Finally, electromyographic recordings of muscles supporting the upper airway and of the primary respiratory muscles will be employed to determine coordination of their activity during gasping. The functional significance of this coordination in "autoresuscitation" from apnea due to upper airway obstruction will be evaluated.